There's something charmingly Quixotic about a Mexican President ambling into the White House in cowboy boots and urging his host to make the most profound change in decades to U.S. immigration law  and to do it before Christmas. But Vicente Fox is nothing if not Quixotic. It took an iconoclast with an irrepressible buccaneering spirit to break the monopoly on power of the Institutional Revolutionary Party, which had run Mexico as its private hacienda for 70 years. But Fox ultimately owed his popular election victory last year to the profound hopes of most Mexicans for a better life. And right now, the former Coca-Cola executive's ability to deliver is dependent in no small part on his special relationship with President George W. Bush.

Fox's ambitious agenda encompasses everything from police reform, stamping out corruption and cutting a political deal to end the Zapatista rebellion in Chiapas to opening up Mexico's heavily bureaucratized economy to the bracing winds of market forces. And right now, he's not doing very well. Nor could he be expected to, since the legislature is controlled by his enemies. Russian reformer Boris Yeltsin had the same problem in the '90s, but Fox doesn't have Yeltsin's option of sending in tanks to shell parliament. So, much of his agenda remains stalled.

As in the case of Yeltsin, Washington has a deep desire to see Fox succeed  and a strong self-interest in doing what it can to help him. President Bush is prepared to go to bat for the rights of Mexican truckers to ride U.S. highways, and to press Congress to abandon the annual ritual of certifying Mexico as a drug war ally, which the Mexicans find insulting. But the faltering U.S. economy is making life difficult for Fox, as last year's roaring 7 percent growth rate gives way to this year's stagnation, and Mexico sheds jobs instead of creating desperately-needed new ones. And that, of course, raises the pressure on poor Mexicans to head north.

President Fox's pushiness in surprising the White House with his call to conclude an immigration deal by year's end is a reminder of how much he had riding on this week's state visit. Immigration is an area where Bush can help Fox a great deal  and even potentially help himself in the process. More than 3 million undocumented Mexicans currently work in the U.S., and Fox wants their status legalized. His reasons are partly economic, and partly political. Granting them legal status would free Mexican migrants from the exploitation of the low-wage off-the-books economy, and therefore expand the earning potential of an expatriate community that will send home an estimated $9 billion this year alone. It would also allow these workers to travel back and forth, which is far too dangerous right now for most of those lucky enough to have eluded the Border Patrol once  and that cements the ties to home, and probably the southward flow of household capital, too. Politically, the current immigration regime is perceived by Mexicans as brutal, inhumane and an assault on their dignity, and that compounds a profound sense of resentment south of the border. The basic message Fox brought to Washington was that his is a new Mexico, and it wants the trust and respect of the U.S.

For Bush, too, there would be benefits. Undocumented workers are often untaxed workers, and bringing 3 million of them onto the books would add a tidy few billion to U.S. tax revenues, too. (And the Bush administration has signaled that any form of legalization should involve the undocumented workers paying a fine for having broken the law until now.) They're already an integral part of the U.S. economy, and trying to get rid of them would be harmful. And raising the status of the undocumented Mexican underclass could also help Bush grow his share of the increasingly-important Latino vote  an estimated 60 percent of U.S. Latinos are of Mexican origin.

The fact that immigration reform is on the table is a sign that Washington has accepted a truth that Mexico has long held to be self-evident: That migration across the Rio Grande is essentially an economic problem. There'd be little reason for Mexicans to risk the harrowing passage if there were decent jobs to be had at home. Thus Fox's plan for immigration reform, coupled with economic reform to grow the Mexican economy. But President Bush also has plenty of reason to be cautious, since curbing immigration has long been a hot-button issue for his party's conservative base. And the U.S. economic downturn will increase the pressure that forced the White House to backpedal on the wide-ranging amnesty it floated at the beginning of summer. Today, the Bush administration is talking not of an amnesty, but of a "guest worker" program.

Still, the very fact that the White House and Congress are ready to seriously entertain discussion over altering immigration laws in favor of illegal migrants from Mexico signals how far the integration of the two economies and societies has progressed in recent years. Many obstacles lie ahead, and the two governments will have substantially different concerns when they get down to negotiating a new immigration deal. Still, the tenor of this week's discussions in Washington suggest a sense of common destiny and shared responsibility unparalleled in the troubled history of U.S.-Mexican relations. And that may be President Fox's greatest achievement to date.